1. Field of the Invention
The present invention relates to an inverter and more particularly, to an inverter for providing AC voltages having two different frequencies to a lamp circuit.
2. Description of the Prior Art
A high voltage lamp is a common optical part found in devices used in daily life such as scanners and fax machines and need high voltage lamps for operation. Normally, a high voltage lamp is powered by an inverter. The duties of the inverter are to first transform a DC voltage into an AC voltage and then provide the AC voltage to drive the high voltage lamp. A characteristic of the high voltage lamp is that the power requirement of the high voltage lamp during a starting state differs from a normal luminescent state; usually the power required during the starting state is larger than the power required during the normal luminescent state. If the amount of power used in the normal luminescent state is used in the starting state to start the high voltage lamp, it will take too much time for the lamp to start up; hence, the operation of the high voltage lamp will become inconvenient.
As mentioned above, a high voltage lamp uses AC voltage. Assume that for a lamp circuit (including a high voltage lamp) to operate in normal luminescent state, an operating AC voltage is needed for providing suitable power to the lamp circuit. In order to provide more power to the lamp circuit during the starting state, typically, a starting AC voltage having a larger amplitude than the operating voltage is provided to the lamp circuit (normally the frequency of the operating AC voltage is the same as the frequency of the starting AC voltage). Theoretically, the power an AC voltage can provide is determined by the amplitude of the AC voltage; the larger the amplitude of the AC voltage, the more power the AC voltage can provide. If the amplitude of the starting AC voltage is larger than the amplitude of the operating AC voltage, then the starting AC voltage can provide more power than the operating AC voltage and allow the lamp circuit to achieve the goal of a fast start up.
Please refer to FIG. 1 a block diagram of a prior art inverter is illustrated. A power supply 110 provides a first DC voltage V1 and a second DC voltage V2. An inverter 120 includes a first switch 130, an oscillating circuit 150, and a transformer 170. The function of the first switch 130 is to selectively pass V1 or V2 to the oscillating circuit 150 as input voltage. The oscillating circuit 150 receives a DC voltage and then oscillates to generate an AC voltage having frequency Fq for outputting. The transformer 170 receives the AC voltage provided by the oscillating circuit 150, transforms the AC voltage, and outputs the transformed AC voltage to the lamp circuit 190, which includes a high voltage lamp 195.
No matter if V1 or V2 serves as the input voltage of the oscillating circuit 150, the frequency of the AC voltage generated by the oscillating circuit 150 will be fixed to the same value (the fixed value is determined by the parameters of elements of the oscillating circuit 150). When the transformer 170 receives the AC voltage provided by the oscillating circuit 150, the frequency of the AC voltage will not be changed by the transformer 170, so the frequency of the AC voltage Vac outputted by the transformer 170 is still Fq.
Assume that when the first DC voltage V1 serves as the input voltage of the oscillating circuit 150, the AC voltage Vac outputted by the transformer 170 has an amplitude Vac1; when the second DC voltage V2 serves as the input voltage of the oscillating circuit 150, the AC voltage Vac outputted by the transformer 170 has an amplitude Vac2. Also assume that the AC voltage Vac with amplitude Vac1 and frequency Fq is the AC voltage suitable for the lamp 195 of the lamp circuit 190 to operate at the normal luminescent state. As mentioned before, in order to provide larger power during the starting state, typically, the amplitude Vac2 must be larger than the amplitude Vac1, and the AC voltage with amplitude Vac2 must be used during the starting state to achieve the goal of a fast start up.
To sum up, in the prior art for starting the lamp 195 of the lamp circuit 190, the second DC voltage V2 is used as input voltage of the oscillating circuit 150. To satisfy the requirement that the amplitude Vac2 is larger than the amplitude Vac1, the second DC voltage V2 must be larger than the first DC voltage V1. The operation principle is that during the starting state, the first switch 130 passes the second DC voltage V2 to the oscillating circuit 150 in order to start the lamp 195 quickly; after the starting state is finished (usually when the lamp 195 has reached 80% of its luminosity), the first switch 130 switches to pass the first DC voltage V1 to the oscillating circuit 150 to operate the lamp 195 at the normal luminescent state.
However, the prior art solution suffers from a few problems. The main problem is that when using the AC voltage with larger amplitude to start up the lamp 195, the current flow through the lamp 195 is also larger than normal operating current. Normal operating current has little effect on the lifetime of the lamp 195, but the larger current used during the starting state usually damages the lamp 195 and thereby reducing the lifetime of the lamp 195. For a high voltage lamp that must be turned on and off frequently, the reduction of the lifetime is a serious problem.
In a device such as a scanner, a fax machine, a high voltage lamp is always a critical component. When the high voltage lamp is damaged, the whole device will also lose normal functionality. So the lifetime of the high voltage lamp in such kind of devices is very important. In conclusion, a main problem of the prior art which uses an AC voltage having larger amplitude to start a high voltage lamp than the amplitude of the AC voltage used during a normal operating state, is that the reduction of the lifetime of the high voltage lamp.